# Simulation of a turbulent flow into a diffuser

 Register Blogs Members List Search Today's Posts Mark Forums Read August 12, 2021, 12:44 Simulation of a turbulent flow into a diffuser #1 New Member   Join Date: Aug 2021 Posts: 4 Rep Power: 2 Hi, I'm running a simulation of a turbulent flow into a plane diffuser. Before the diffuser there is a reptangular duct, used to simulate fully developed flow. I'm using a SST model, with a second order upwind scheme. I've tried different ways to solve my problem, the last was using a first order discretisation for turbulent variable and a second order discretisation scheme for convective variable with limiter function, but I' can't converge to a feasibile solution. The solution seems asymmetrical respect to the axis. This is my configuration file. % ------------- DIRECT, ADJOINT, AND LINEARIZED PROBLEM DEFINITION ------------% % % Physical governing equations (EULER, NAVIER_STOKES, % WAVE_EQUATION, HEAT_EQUATION, FEM_ELASTICITY, % POISSON_EQUATION) SOLVER= INC_RANS % % If Navier-Stokes, kind of turbulent model (NONE, SA) KIND_TURB_MODEL= SST % % Mathematical problem (DIRECT, CONTINUOUS_ADJOINT) MATH_PROBLEM= DIRECT % % Restart solution (NO, YES) RESTART_SOL= NO % ---------------- INCOMPRESSIBLE FLOW CONDITION DEFINITION -------------------% % % Initial density for incompressible flows (1.2886 kg/m^3 by default) INC_DENSITY_INIT= 1.225 % % Initial velocity for incompressible flows (1.0,0,0 m/s by default) INC_VELOCITY_INIT= ( 0.598767, 0.0, 0.0 ) % % List of inlet types for incompressible flows. List length must % match number of inlet markers. Options: VELOCITY_INLET, PRESSURE_INLET. INC_INLET_TYPE= VELOCITY_INLET % % Damping coefficient for iterative updates at pressure inlets. (0.1 by default) INC_INLET_DAMPING= 0.1 % % List of outlet types for incompressible flows. List length must % match number of outlet markers. Options: PRESSURE_OUTLET, MASS_FLOW_OUTLET INC_OUTLET_TYPE= PRESSURE_OUTLET % % Damping coefficient for iterative updates at mass flow outlets. (0.1 by default) INC_OUTLET_DAMPING= 0.1 % --------------------------- VISCOSITY MODEL ---------------------------------% % % Viscosity model (SUTHERLAND, CONSTANT_VISCOSITY). VISCOSITY_MODEL= CONSTANT_VISCOSITY % % Molecular Viscosity that would be constant (1.716E-5 by default) MU_CONSTANT= 1.7894e-05 % % ---------------------- REFERENCE VALUE DEFINITION ---------------------------% % % Reference origin for moment computation REF_ORIGIN_MOMENT_X = 0.25 REF_ORIGIN_MOMENT_Y = 0.00 REF_ORIGIN_MOMENT_Z = 0.00 % % Reference length for pitching, rolling, and yawing non-dimensional moment REF_LENGTH= 1.0 % % Reference area for force coefficients (0 implies automatic calculation) REF_AREA= 2.0 % % -------------------- BOUNDARY CONDITION DEFINITION --------------------------% % % Navier-Stokes wall boundary marker(s) (NONE = no marker) MARKER_HEATFLUX= ( wall_1, 0.0 , wall_2, 0.0, wall_3, 0.0, wall_4, 0.0) % % Inlet boundary marker(s) (NONE = no marker) % Format: ( inlet marker, total temperature, total pressure, flow_direction_x, % flow_direction_y, flow_direction_z, ... ) MARKER_INLET= ( inlet, 0.0, 0.598767 , 1.0, 0.0, 0.0 ) % % Outlet boundary marker(s) (NONE = no marker) % Format: ( outlet marker, back pressure, ... ) MARKER_OUTLET= ( outlet, 0.0) % % Marker(s) of the surface to be plotted or designed MARKER_PLOTTING= ( wall_1,wall_2, wall_3, wall_4) % % Marker(s) of the surface where the functional (Cd, Cl, etc.) will be evaluated MARKER_MONITORING= ( wall_1,wall_2 ) % FREESTREAM_TURBULENCEINTENSITY= 0.01 % FREESTREAM_TURB2LAMVISCRATIO= 3 % % ------------- COMMON PARAMETERS DEFINING THE NUMERICAL METHOD ---------------% % % Numerical method for spatial gradients (GREEN_GAUSS, LEAST_SQUARES, % WEIGHTED_LEAST_SQUARES) NUM_METHOD_GRAD= GREEN_GAUSS % % Courant-Friedrichs-Lewy condition of the finest grid CFL_NUMBER= 10 % % Adaptive CFL number (NO, YES) CFL_ADAPT= NO % % Parameters of the adaptive CFL number (factor down, factor up, CFL min value, % CFL max value ) CFL_ADAPT_PARAM= ( 0.1, 2.0, 100.0, 1e3 ) % % Runge-Kutta alpha coefficients RK_ALPHA_COEFF= ( 0.66667, 0.66667, 1.000000 ) % % Number of total iterations ITER= 99999 % ------------------------ LINEAR SOLVER DEFINITION ---------------------------% % % Linear solver for implicit formulations (BCGSTAB, FGMRES) LINEAR_SOLVER= FGMRES % % Preconditioner of the Krylov linear solver (JACOBI, LINELET, LU_SGS) LINEAR_SOLVER_PREC= ILU % % Linael solver ILU preconditioner fill-in level (0 by default) LINEAR_SOLVER_ILU_FILL_IN= 0 % % Minimum error of the linear solver for implicit formulations LINEAR_SOLVER_ERROR= 1E-12 % % Max number of iterations of the linear solver for the implicit formulation LINEAR_SOLVER_ITER= 20 % % -------------------------- MULTIGRID PARAMETERS -----------------------------% % % Multi-Grid Levels (0 = no multi-grid) MGLEVEL= 0 % % Multi-grid cycle (V_CYCLE, W_CYCLE, FULLMG_CYCLE) MGCYCLE= V_CYCLE % % Multi-grid pre-smoothing level MG_PRE_SMOOTH= ( 1, 1, 1, 1 ) % % Multi-grid post-smoothing level MG_POST_SMOOTH= ( 0, 0, 0, 0 ) % % Jacobi implicit smoothing of the correction MG_CORRECTION_SMOOTH= ( 0, 0, 0, 0 ) % % Damping factor for the residual restriction MG_DAMP_RESTRICTION= 0.8 % % Damping factor for the correction prolongation MG_DAMP_PROLONGATION= 0.8 % % -------------------- FLOW NUMERICAL METHOD DEFINITION -----------------------% % % Convective numerical method (JST, LAX-FRIEDRICH, CUSP, ROE, AUSM, HLLC, % TURKEL_PREC, MSW) CONV_NUM_METHOD_FLOW= FDS % % Monotonic Upwind Scheme for Conservation Laws (TVD) in the flow equations. % Required for 2nd order upwind schemes (NO, YES) MUSCL_FLOW= YES % % Slope limiter (NONE, VENKATAKRISHNAN, VENKATAKRISHNAN_WANG, % BARTH_JESPERSEN, VAN_ALBADA_EDGE) SLOPE_LIMITER_FLOW= VENKATAKRISHNAN % % Time discretization (RUNGE-KUTTA_EXPLICIT, EULER_IMPLICIT, EULER_EXPLICIT) TIME_DISCRE_FLOW= EULER_IMPLICIT % -------------------- TURBULENT NUMERICAL METHOD DEFINITION ------------------% % % Convective numerical method (SCALAR_UPWIND) CONV_NUM_METHOD_TURB= SCALAR_UPWIND % % Monotonic Upwind Scheme for Conservation Laws (TVD) in the turbulence equations. % Required for 2nd order upwind schemes (NO, YES) MUSCL_TURB= NO % % Slope limiter (VENKATAKRISHNAN, MINMOD) SLOPE_LIMITER_TURB= VENKATAKRISHNAN % % Time discretization (EULER_IMPLICIT) TIME_DISCRE_TURB= EULER_IMPLICIT % ----------------------- SLOPE LIMITER DEFINITION ----------------------------% % % Coefficient for the limiter VENKAT_LIMITER_COEFF= 0.05 % % Coefficient for the sharp edges limiter ADJ_SHARP_LIMITER_COEFF= 3.0 % % Reference coefficient (sensitivity) for detecting sharp edges. REF_SHARP_EDGES= 3.0 % % Remove sharp edges from the sensitivity evaluation (NO, YES) SENS_REMOVE_SHARP= NO % --------------------------- CONVERGENCE PARAMETERS --------------------------% % % Convergence criteria (CAUCHY, RESIDUAL) CONV_FIELD= RMS_PRESSURE % % Min value of the residual (log10 of the residual) CONV_RESIDUAL_MINVAL= -13 % % Start convergence criteria at iteration number CONV_STARTITER= 10 % % Number of elements to apply the criteria CONV_CAUCHY_ELEMS= 100 % % Epsilon to control the series convergence CONV_CAUCHY_EPS= 1E-6 % % ------------------------- INPUT/OUTPUT INFORMATION --------------------------% % % Mesh input file MESH_FILENAME= dominio_completo_M1.su2 % % Mesh input file format (SU2, CGNS, NETCDF_ASCII) MESH_FORMAT= SU2 % % Mesh output file MESH_OUT_FILENAME= mesh_out.su2 % % Restart flow input file SOLUTION_FILENAME= solution_flow.dat % % Restart adjoint input file SOLUTION_ADJ_FILENAME= solution_adj.dat % % Output file format (PARAVIEW, TECPLOT, SLT) TABULAR_FORMAT= CSV % % Output file convergence history (w/o extension) CONV_FILENAME= history % % Output file restart flow RESTART_FILENAME= restart_flow.dat % % Output file restart adjoint RESTART_ADJ_FILENAME= restart_adj.dat % % Output file flow (w/o extension) variables VOLUME_FILENAME= flow_M1 % % Output file adjoint (w/o extension) variables VOLUME_ADJ_FILENAME= adjoint % % Output objective function gradient (using continuous adjoint) GRAD_OBJFUNC_FILENAME= of_grad.dat % % Output file surface flow coefficient (w/o extension) SURFACE_FILENAME= surface_flow_M1 % % Output file surface adjoint coefficient (w/o extension) SURFACE_ADJ_FILENAME= surface_adjoint % % Writing solution file frequency WRT_SOL_FREQ= 250 % % Writing convergence history frequency WRT_CON_FREQ= 1 % % Screen output SCREEN_OUTPUT= (INNER_ITER, WALL_TIME, RMS_PRESSURE, RMS_NU_TILDE, LIFT, DRAG) Anyone can help me? I can't find my mistake Last edited by Sam6789; August 13, 2021 at 04:01.   August 13, 2021, 17:42 #2 Member   bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 71 Rep Power: 12 What does the convergence look like? Does it converge but to a solution that you do not expect to be physically correct? Or does it not converge at all? If it does not converge at all, then the first thing to check is the mesh quality. Also check if the inner iterations converge within 20 iterations with LINSOL_ITER and LINSOL_RESIDUAL in the SCREEN_OUTPUT. Also try with the SA turbulence model.  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